1. Field of the Invention
This application relates to biasing circuitry for MEMS transducers, in particular to biasing circuitry comprising a reference circuit such as a bandgap reference circuit and a DC-DC converter such as a charge pump.
2. Description of the Related Art
MEMS transducers such as MEMS capacitive transducers, for instance MEMS microphones, typically comprise two plates which are movable with respect to one another, for example a fixed plate and a moveable membrane. A stimulus, such as an acoustic pressure wave in the case of a microphone, can vary the distance between the plates of the MEMS transducer resulting in a capacitance that varies in accordance with the stimulus. In use a bias voltage is typically applied across the varying capacitance to provide a consequently varying electrical signal voltage or charge which can be measured. For MEMS transducers the overall capacitance of the transducer is small, typically of the order of 1 pf or so, and the change in capacitance is typically less than 1%. Thus a sensitive low-noise preamplifier is required to buffer the measured signal, i.e. the signal from the transducer.
MEMS microphone transducers are typically designed to require a bias voltage of around 12V. This is larger than the power supply voltages used for amplifiers and other electronic circuitry, so the bias voltage may be generated and supplied by a suitable DC-DC converter, typically a switched DC-DC converter such as a charge pump.
FIG. 1 illustrates an example of a typical arrangement of pre-amplifier circuitry for a MEMS sensor 100. A first terminal of MEMS transducer 101 is arranged to receive a bias voltage VCP, typically 12V or so, from a charge pump 102. An amplifier 103 has an input connected to the other terminal of the MEMS transducer. This terminal is also connected to a high-value (typically of the order of 10 Gohm or greater) bias resistance RG 104 to bias this terminal to ground without shorting out the audio band signal. The bias resistance 104 may often be implemented in the form of polysilicon diodes.
The charge pump 102 is arranged to generate the required relatively high bias voltage VCP from a lower voltage input. Typically the charge pump 102 generates a bias voltage VCP which is equal to a multiple of the voltage applied to its input. It will be appreciated therefore that were the charge pump input connected directly to the voltage supply for an integrated circuit, then the bias voltage across the transducer would vary with the applied supply voltage. Also any noise on the supply would be similarly multiplied and couple via the MEMS capacitance into the amplifier 103 and would be indistinguishable from any acoustically generated signals. Thus the voltage input for the charge pump 102 is preferably a supply-independent voltage VR. Typically this reference voltage VR is generated by a reference generator circuit 105 which will typically include a bandgap voltage reference generator. As will be understood by one skilled in the art a bandgap voltage reference generator can generate a reference voltage that is independent of variations of the supply voltage and which is also substantially temperature stable.
The power supply rejection of the amplifier circuit is also important to avoid coupling of supply noise into the signal path, so preferably the amplifier 103 is supplied with a supply-independent reference current, i.e. bias current IB, which also advantageously allows the supply current to be optimised without having to allow extra margin for the tolerance in supply voltage. Conveniently the reference voltage VR and the bias current IB are both supplied from the same reference generator circuit 105.
FIG. 2 illustrates one example of bandgap reference generation circuitry 105. To provide the bandgap reference voltage VBG, bipolar transistors Q1 and Q2 are configured to run at different current densities J1, J2, giving rise to a difference in their base-emitter voltages, Vbe, equal to (kT/q)·ln(J2/J1). Control circuitry 201 equalises the voltages at nodes A and B, resulting in this temperature-proportional voltage being imposed across a resistor R1, which thus passes a current IPTAT which is proportional to temperature. The equality is possible for only one non-zero current IPTAT through node A and the corresponding current through node B, which the control circuitry establishes by controlling at least one current source 202 to deliver a current IXA. This current is supply-independent, and thus may be mirrored to provide a supply-independent output current IXB for use by the amplifier circuit 103.
The base-emitter voltage Vbe of Q2 (or Q1) has a physically determined negative temperature coefficient of magnitude about 2 mV/K. The circuit is configured so that the negative temperature dependence of the transistor base-emitter voltage Vbe at least partially offsets a positive temperature coefficient due to the temperature-proportional current flowing through appropriate resistances. In other words, through choice of an appropriate resistive scaling factor Rx and current IPTAT, an output voltage VBG=Vbe+Rx·IPTAT is provided with a zero (or at least smaller) net temperature coefficient.
Such a bandgap reference circuit 105 thus provides a supply independent current and a supply independent voltage VBG that is relatively temperature stable. It is not typically possible however to use such a reference voltage VBG directly as an input to a charge pump as the charging current pulses drawn by the charge pump cannot be supplied without disturbing the operation of the bandgap reference circuit 105. Typically therefore the reference voltage VBG which is produced by the bandgap reference voltage generator 105 may be buffered by a suitable voltage buffer and this buffered reference voltage VBBG is provided as the input voltage to the charge pump.
In MEMS sensor applications however there are often pressures on the cost and size of the sensor circuitry and thus it would be desirable to be able to reduce the size of the sensor circuitry where possible.
Such MEMS sensors are also often used in battery powered device where power consumption is always a concern, especially in the case of MEMS microphones which may be used for relatively long periods of time, for instance to allow for input of voice commands. Thus, the supply current for a pre-amplifier circuit for a MEMS microphone is an important consideration and should be kept as low as possible.
Embodiments of the present invention therefore provide biasing circuitry for MEMS transducers that address at least some of the issues mentioned above.